Does variscite control phosphate availability in acidic natural waters? An experimental study of variscite dissolution rates

摘要

The dissolution rates of natural, well crystallized variscite (AlPO_4·2H_2O) were determined from the evolution of aqueous Al and P concentrations in closed and open-system mixed-flow reactors at 25°C and pH from 1.5 to 9.0. Measured dissolution rates decrease with increasing pH, from 6×10~(-16)mol/cm~2/s at pH 1.5 to 5×10~(-17)mol/cm~2/s at pH 5.89, and then increase with increasing pH to 4×10~(-16)mol/cm~2/s at pH 9.0. Geochemical modeling calculations, performed using measured dissolution rates, indicate that it would take no more than a few weeks or months to equilibrate a mildly acidic, Al and P-free solution with variscite. Hence, variscite can buffer aqueous phosphate concentrations in mildly acidic near surface environments. This conclusion is confirmed by consideration of the compositions of natural waters.